Climatology of Lake Nasser in Egypt

Transcription

1 Middle East Journal of Applied Sciences Volume : 08 Issue :03 July-Sept Pages: Climatology of Lake Nasser in Egypt Mohamed I. EL-Mekawy 1, Zeinab Salah 1 and M. M. Abdel Wahab 2 1 Egyptian Meteorological Authority, Cairo, Egypt. 2 Astronomy, Space Sciences and Meteorology Dept., Faculty of Sciences, Cairo, Univ., Egypt. Received: 03 June 2018 / Accepted: 04 July 2018 / Publication date: 15 July 2018 ABSTRACT Lake Nasser in south Egypt extends over 500 km, 350 km in Egypt and 150 km in Sudan. It has an average width of about 12 km at the 180 m water level. However, as the capacity of Lake Nasser to store water is not without limits, it is of the outmost importance for Egypt to understand properly the changes, which occur in this water body, in this work we discuss two important issues. The first one is the climate of the region where the lake is located have been changed after the lake construction. The most significant parameters affecting evaporation from the lake in order to search for adaptation strategy to reduce evaporation. This work reveals that no significant changes for the climate due to lake construction and the temperature at 2 m high is the main contributing factor to evaporation loses. Keywords: Nasser Lake, climate, evaporation, ERA-20C, ERA-Interim, Aswan, Abu-Simbel Introduction Building large dams and their artificial lakes can lead to significant changes in the ecosystem of the surrounding area, as they increase the area of water surface exposed to evaporation. In addition to thechanges in the land-use and land cover can lead to significant impacts on the temperature and precipitation the regions surrounding the dams (Degu et al., 2011; and Deng et al., 2013).The influences of the dams and their lakes on the precipitation patterns, focusing on the extreme events, have been investigated in various studies; which found anenhancement of the precipitation intensity happen after reservoir construction in the regions close to them (Hossain et al., 2010; Yigzaw et al., 2012; and Pizarro et al., 2013). Gangoiti et al. (2011) revealed that terrestrial evaporation from the lakes of the reservoirs could be a trigger of the extreme precipitation. Moreover, dams can increase the convective available potential energy (CAPE), producing more precipitation in the areas close to reservoirs in the Mediterranean and arid climate zones of the United States (Degu et al., 2011). However Lü et al. (2018) did not find a significant influence of the Three Gorges Dam in China on the extreme precipitation, relative humidity and water vapor pressure. The High Dam in Egypt is one of the greatest reservoirs in the world. It was constructed to the south of Aswan during the period from January 1964 to June 1968, to manage the water of the Nile flood, and to generate low-cost hydroelectric power for Egypt, in addition to storing the Nile flows in the lake behind it, which called Lake Nasser, which is considered as the water bank of Egypt. This study is an attempt to detect the influence of the High Dam and Lake Nasser on the regional climate, through comparing some meteorological parameters pre and post Lake s creation, over two cities of Aswan and Abu-Simbel, located in the north and the south of Egyptian part of the Lake, respectively. We compared the changes of the air temperature, dew point temperature, and wind, available from reanalysis data. Moreover, we tried to find the most significant meteorological parameter affecting on the evaporation process, to understand the effect of the climate change on LakeNasser water content and to find a way to reduce the water loss from the Lake. The paper is organized as follows: Section 2 presents the study region, the datasets used in this study and the methods used for calculations of relative humidity, and evaporation. The results are shown and discussed in Section 3, and finally the conclusions and future work are reported in Section 4. Corresponding Author: Mohamed I. EL-Mekawy, Egyptian Meteorological Authority, Cairo, Egypt. mohamedmekawy26@gmail.com 719

2 Fig. 1: Map of Egypt with blue circle around the Lake Nasser and the cities of Aswan and Abu- Simbel in the north and south of Lake Nasser in the Egyptian part, respectively. Study region, data and methodology: Lake Nasser occupies parts of southern Egypt and northern Sudan. This study concerns about the Egyptian part, located between Latitudes 22 and 24 N and Longitudes 31 and 34 E (Fig.1), extended about 500 km in length until Aswan. The total capacity of the lake is 162.3x10 9 m 3 from fresh water and renewable at level of 182 m. The mean width of the lake is about 10 km with a maximum value of 60 km, and the mean depth is about 25 m with maximum value of 90 m (Sadek et al., 1997). The surface area of Lake Nasser is variable due to the annual amount of flood entering the lake and the water discharges from it. When the High Dam is nearly full with water level of 180 m, the lake surface area could exceed 6276 km 2 (Jeongkon and Mohamed, 2002), with shoreline of more than 7800 km length (Hamdan and Zaki, 2016). In this study we used two types of reanalysis data from ECMWF; the first one is ECMWF atmospheric reanalysis for the 20 th century, from (ERA-20C), and the other one is the ERA-Interim, both of them available on ( ERA-20C is the atmospheric reanalysis produced by ECMWF for the 20th century from 1900 to ERA-20C, which is a product of the ERA-CLIM project, assimilates only the surface pressure and surface marine winds observations. This dataset was produced using the recent version of ECMWF s Integrated Forecast System (IFS) with the assimilation method of 24-hour 4-dimensional variational analysis (4D-Var). The horizontal resolution is approximately 125 km, on the native 91 model levels (from the surface to 0.01 hpa), and 37 pressure levels (from 1000 to 1 hpa), in addition to 16 potential temperature levels, and the 2 PVU potential vorticity levels (Poli et al., 2013 and Hersbach, et al., 2015). ERA-Interim, is a global atmospheric reanalysis produced by ECMWF, but from 1979, continuously updated in real time. The data assimilation system used in producing this dataset is a 2006 version of the IFS (Cy31r2). The system includes a 4D-Var with a 12-hour analysis window. The spatial resolution of ERA-Interim is approximately 80 km, with native 60 vertical model levels (from the surface up to 0.1 hpa), and 37 pressure levels (Berrisford et al. 2011). 720

3 To calculate the evaporation rate, one can use equation 1, which represents the bulk Aerodynamic method with some modifications added by the (Hassan et al., 2007). As this method always gives a value for evaporation even if the air velocity is equal to zero. E = 0.13 ( U 2) (e s e a) eq.(1) Where E is the evaporation rate (mm/day), U 2 is the wind velocity (m/sec) at 2.0 m height above water, es is the saturated vapour pressure (in hpa) at water surface temperature, and ea is the actual vapour pressure of the air (in hpa). For the purpose of our study, we used the monthly means of air temperature and dew point temperature at 2 m, and the zonal and meridional wind components at 10 m, with spatial resolution of x degree, and then we extracted the values over the two stations of Aswan and Abu- Simbel. The studied period was divided into three periods as the following, with taking the total period of as the reference period. 1- The period ( ) before construction of Nasser Lake, hereinafter referred as Period 1, from ERA-20C. 2- The period ( ) after construction of the lake, hereinafter referred as Period 2, from ERA-20C. 3- The period ( ), referred as Period 3, which represents the current climate, from ERA- Interim. To study which parameters from the air temperature, relative humidity, and wind speed has the most significant effect on the evaporation rate based on equation 1, we assumed increase each parameter by 10% from its value with keeping the others constant, and calculate the evaporation rate to evaluate the difference between before and after the changes. Results and Discussion In this section, we will introduce the analyses of the climatology parameters at Abu-Simbel and Aswan, through comparing of the air temperature, dew point temperature, calculated relative humidity, the wind speed and the calculated evapotranspiration before direction before lake construction (period 1),after construction of the High Dam(period 2) and period 3 to describe the current situation. Climatology of Aswan Table 1, includes the seasonal means of the air temperature (AT), dew point temperature (DPT) and the relative humidity (RH) over Aswan and Abu-Simbel, during the whole period from 1955 to 2013, which will referred as the reference values. Figure 2, shows the anomalies of the seasonal means of AT (in blue bars), DPT (in red bars), and RH (in green bars), with their trend lines in the same colours over Aswan, calculated for the three periods. As one can notice during the summer (Figure 2, on the upper left); the AT, DPT, and RH have negative anomalies during the period1 (pre-lake) and period2 (post-lake); since after Nasser Lake creation, the temperature and dew point decreased more than before the construction, but the relative humidity increased slightly in the second period. Whereas in the third period all of them have positive anomalies, reached to more than 4 degrees in DPT. In the autumn (Figure 2, on the upper right); the anomalies of AT and DPT became more negativity in the 2 nd period, whereas the anomaly of RH converted from positive in period 1 to be negative after construction of the lake. However in the 3 rd period, all of AT and DPT increased than their reference values (in Table1) by more than 2 degrees with increasing trends, while the anomaly of RH is approximately 1 in addition to its trend seems to be stable. In contrary to the summer and autumn anomalies in the 1 st period, all parameters in the winter (in the lower left chart), have positive anomalies. However, in the 2 nd period all of them decreased as in the summer and the autumn. In the 3 rd period AT and DPT increased with slightly positive trend but RH decreased with a negative trend. Finally, the changes in the anomalies during the spring (lower right chart in Figure 2) are approximately similar to those in the summer, this could be due to the slightly differences in the air temperature and humidity between the two seasons in Egypt. 721

4 Table 1: The seasonal mean values of the air temperature (AT), dew point temperature (DPT) (in K), and Relative humidity (RH, in %), calculated during the period of , over Aswan and Abu-Simbel. Aswan Abu-Simbel Season AT (k) DPT (k) RH (%) Summer Autumn Winter Spring Season AT (k) DPT (k) RH (%) Summer Autumn Winter Spring Fig. 2: The anomalies calculated over Aswan, for of the seasonal means of the air temperature (AT; in blue bars), the dew point temperature (DPT; in red bars), and the relative humidity (RH; in green bars), with their trend lines in the same colours. The anomalies were calculated during the three periods as illustrated in the text relative to the reference period ( ), for the summer (in the upper left figure), autumn (in the upper right), winter (in the lower left), and spring (in the lower right). From the wind direction and the frequencies distribution mentioned in Table 2, it can be noticed that the prevailing wind over Aswan ranges between northerly and north westerly during most of the year, and the wind speed changes from season to another with the range of 2-5 m/s in the majority of the year. Only in the summer and spring of period of , the mean wind speed exceeded 5.7 m/s in approximately 28% and 1.6%, respectively, of the available data, whereas in the other two periods, the wind speed was lower than 5.7 m/s. Climatology of Abu-Simbel Figure 3 shows the anomalies of the seasonal means of AT, DPT, and RH, with their trend lines, as in Figure 2, but for Abu-Simbel. Approximately the anomalies of Abu-Simbel are similar to those of Aswan, with little differences in their quantities in all seasons. 722

5 Table 2: Wind direction (blowing from), Wind classes (in m/s) and their frequencies distribution in percent, in the four seasons in each period, over Aswan. Season Summer Autumn Winter Spring a. NW-N b. 27% with wind speed of c. 44.4% with wind speed of d. 28.6% with wind speed of b. 52.4% with wind speed of c. 47.6% with wind speed of b. 74.6% with wind speed of c. 25.4% with wind speed of b. 31.7% with wind speed of c. 66.7% with wind speed of d. 1.6 % with wind speed of a. NW b. 36.5% with wind speed of c. 63.5% with wind speed of b. 57.1% with wind speed of c. 42.9% with wind speed of b. 61.9% with wind speed of c. 38.1% with wind speed of b. 71.4% with wind speed of c. 28.6% with wind speed of a. NW b. 19.6% with wind speed of c. 80.4% with wind speed of b. 43.1% with wind speed of c % with wind speed of b. 78.4% with wind speed of c. 21.6% with wind speed of a. NW-N b. 56.9% with wind speed of c. 43.1% with wind speed of Fig. 3: The anomalies calculated over Abu-Simbel, for of the seasonal means of the air temperature (AT; in blue bars), the dew point temperature (DPT; in red bars), and the relative humidity (RH; in green bars), with their trend lines in the same colours. The anomalies were calculated during the three periods as illustrated in the text relative to the reference period ( ), for the summer (in the upper left figure), autumn (in the upper right), winter (in the lower left), and spring (in the lower right). 723

6 From the wind directions and the frequencies distribution mentioned in Table 3, it can be noticed that the prevailing wind over Abu-Simbel blowing from different directions ranges between northern west and northern east during most of the year, through the three periods, and the wind speed changes from season to another with the range of 2-5 m/s in the majority of the year. In the summer of the1 st period of , the mean wind speed exceeded 5.7 m/s in approximately 27%, whereas in the 2 nd period ( ), the frequency of this wind speed was only about 1.6% of the available data. Moreover, this highest wind speed (>5.7 m/s) appeared in the seasons of the autumn and spring only in the 1 st period with frequencies of 1.6% and 7%, respectively. Table 3: Wind direction (blowing from), Wind classes (in m/s) and their frequencies distribution in percent, in the four seasons in each period, over Abu-Simbel. Season Summer Autumn Winter Spring b. 11.1% with wind speed of c % with wind speed of d. 27 % with wind speed of b. 12.7% with wind speed of c. 85.7% with wind speed of d. 1.6 % with wind speed of b. 22.2% with wind speed of c. 77.8% with wind speed of b. 6.3% with wind speed of c. 86.7% with wind speed of d. 7 % with wind speed of Changing in evaporation rate b. 14.3% with wind speed of c % with wind speed of d. 1.6 % with wind speed of b. 4.8% with wind speed of m/s c. 95.2% with wind speed of m/s b. 9.5% with wind speed of m/s c. 90.5% with wind speed of m/s a. N-NE b. 15.9% with wind speed of c. 84.1% with wind speed of m/s a. NW b. 41.2% with wind speed of c % with wind speed of b. 15.7% with wind speed of c % with wind speed of a. N-NE b. 39.2% with wind speed of c. 60.8% with wind speed of a. N-NE b. 47.1% with wind speed of c. 52.9% with wind speed of From the following Table (4), which illustrates the monthly mean of evaporation rate for the three periods calculated over Aswan, on the left columns, and Abu-Simbel on the right columns, one can see that evaporation rate decreased for the second period (Evap-2) respect to that for the 1 st period (Evap-1), then increased for the third one (Evap-3) as trend for rate of evaporation increased and it synchronized with increasing of temperature trend and decreasing of wind speed as shown before, for both of Aswan and Abu-Simbel. Evaluation of impact of the different parameters on the evaporation rate To study which parameter is the most significant one, an increase of about 10% from the mean of each parameter of temperature, wind and relative humidity was added to each parameter, and then the evaporation rate was calculated for the total period of From the result illustrated in Table 5, one can assume that the air temperature at 2 m is the most significant parameter affecting the evaporation rate to be increased by about 21% compared to the evaporation without change over both of Aswan and Abu-Simbel. Then the wind speed was the second significant parameter by increasing 724

7 the evaporation rate by about 6%. While the increasing of the temperature and wind speed lead to increase the evaporation rate, the increasing of the relative humidity leads to decreasing of the evaporation rate by about less than 0.4%from the original value, which agrees with what found by Lü et al. (2018) in the case of the Three Gorges Damin China. Table 4: Monthly mean of evaporation rate calculated using equation (1) over Aswan (on the left) and Abu-Simbel (on the right), during the three periods: (Evap-1), (Evap-2), and (Evap-3). Aswan Abu-Simbel Month Evap-1 Evap-2 Evap-3 Evap-1 Evap-2 Evap-3 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Average Table 5: Monthly mean of evaporation rate over Aswan (on the top) and Abu-Simbel (on the bottom), during total period The 2 nd left column contains the calculated evaporation rate using equation (1) without change, the 3 rd, 4 th and 5 th columns contain the evaporation rates after increasing of 10% from air temperature, wind at 2 m, and the relative humidity, respectively. These columns contain the absolute values after changes and their percentages. Aswan Evaporation (AT+10%) Evaporation (U2+10%) Evaporation (RH+10%) Evaporation Month Value Percent Value Percent Value Percent JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Average Abu-Simbel Evaporation Evaporation (AT+10% ) Evaporation (U2+10%) Evaporation (RH+10%) Month Value Percent Value Percent Value Percent JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Average

8 Conclusion This work studied the effects of the Lake Nasser on the air temperature, dew point temperature, relative humidity, and the wind over the cities of Aswan and Abu-Simbel, and these are the summary of the important results: 1-Both stations near to the lake did not show significant difference in temperature, humidity, wind speed and wind direction. 2-the construction of the lake induced a decreased in temperature by about (2.4ᵒ) in winter and a little increased in humidity in fall by (3 %). 3-Analysis provided the sensitivity of evaporation rate to temperature as the most significant parameter. References Berrisford, P., D.P. Dee, P. Poli, R. Brugge, M. Fielding, M. Fuentes, P.W. Kållberg, S. Kobayashi, S. Uppala and A. Simmons, The ERA-Interim archive Version 2.0, ERA Report Series, 1. Degu, A. M., F. Hossain, D. Niyogi, R. Pielke Sr., J. M. Shepherd, N. Voisin and T. Chronis, The influence of large dams on surrounding climate and precipitation patterns, Geophys. Res. Lett., 38, L04405, doi: /2010GL Deng, X., C. Zhao and H. Yan, Systematic modeling of impacts of land use and land cover changes on regional climate: A review. Advances in Meteorology, Volume 2013, ID , Gangoiti, G., I. Gómez-Domenech and E. Sáez de Cámara, Origin of the water vapor responsible for the European extreme rainfalls of August 2002: 2. A new methodology to evaluate evaporative moisture sources, applied to the August central European rainfall episode. J. Geophys. Res. Atmos., 116, D Hamdan, A. M., and M. Zaki, Long-term estimation of water losses through evaporation from water surfaces of Nasser Lake Reservoir, Egypt. International Journal of Civil & Environmental Engineering IJCEE-IJENS, Vol: 16, No: Hassan, R.M.A., N.T.H. Hekal and N.M.S. Mansor, Evaporation reduction from Lake Nasser using new environmentally safe techniques. Eleventh International Water Technology Conference, IWTC Sharm El-Sheikh, Egypt. Hersbach, H., P. Poli and D.P. Dee, The observation feedback archive for the ICOADS and ISPD data sets, ERA Report Series,18. Hossain, F., I. Jeyachandran and R. Pielke Sr., Dam safety effects due to human alteration of extreme precipitation. Water Resour. Res., 46, W Jeongkon, K. and S. Mohamed, Assessment of long-term hydrologic impacts of Lake Nasser and related irrigation projects in Southwestern Egypt. Elsevier Science, Journal of Hydrology, Lü, M., Y. Jiang, X. Chen, J. Chen, S. Wu and J. Liu, Spatiotemporal Variations of Extreme Precipitation under a Changing Climate in the Three Gorges Reservoir Area (TGRA). Atmosphere, 9, 24; doi: /atmos Pizarro, R., P. Garcia-Chevesich, R. Valdes, F. Dominguez, F. Hossain, P. Ffolliott, C. Olivare, C. Morales, F. Balocchi and P. Bro, Inland water bodies in Chile can locally increase rainfall intensity. J. Hydrol., 481, Poli, P., H. Hersbach, D.G.H. Tan, D.P. Dee, J.N. Thépaut, A. Simmons, C. Peubey, P. Laloyaux, T. Komori, P. Berrisford, R. Dragani, Y. Trémolet, E.V. Hólm, M. Bonavita, L. Isaksen and M. Fisher, The data assimilation system and initial performance evaluation of the ECMWF pilot reanalysis of the 20th-century assimilating surface observations only (ERA-20C), ERA Report Series, 14. Sadek, M.F., M.M. Shahin and C.J. Stigter, Evaporation from the reservoir of the high Aswan dam, Egypt: a new comparison of relevant methods with limited data. Theor Appl Climatol., 56, Yigzaw, W., F. Hossain and A. Kalyanapu, Impact of artificial reservoir size and land use/land cover patterns on probable maximum precipitation and flood: Case of Folsom Dam on the American River. J. Hydrol. Eng., 18,